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J. R. Schrieffer

Publications and source records attributed to J. R. Schrieffer.

16 recordsLinked to original sources
arXiv2004

Gauge Theory Pairing and Spin Fluctuations Near the Quantum Critical Point

We develop a new theory of pairing and magnetic spin fluctuation effect near the quantum critical point. Several novel properties are predicted: 1) based on a spin fermion model, we derive two new interactions, a) a spin deformational potential $H_{sdp}$ proportional to the bandwidth $W$ (as opposed to the considerably smaller exchange coupling $J$ of the nearly antiferromagnetic Fermi liquid theory) and b) a diamagnetic potential $H_{dia}$, quadratic in a gauge potential $\vec{A}$. A dramatic increase of $T_C$ is predicted for $0.01 W\leq J\leq 10 W$. This should have immense technological impact in electric energy production, storage and transmission, as well as for medical electronics, microwave electronics, computer memory and information storage, separations technology and maglev, amongst others. The striking prediction to be confirmed by experiment is that the pairing order parameter $Δ(\vec{k})$ is predicted to be $p-$wave, i.e., $l=1$, S=1, as compared to $l=2$ and S=1 for conventional HTS materials. In addition a novel collective model is predicted whose frequency, $ω_L$ is in the optical range and is determined by $H_{sdp}$.

cond-mat.str-el↗
arXiv2002

Pairing fluctuation theory of high $T_c$ superconductivity in the presence of nonmagnetic impurities

The pseudogap phenomena in the cuprate superconductors requires a theory beyond the mean field BCS level. A natural candidate is to include strong pairing fluctuations, and treat the two-particle and single particle Green's functions self-consistently. At the same time, impurities are present in even the cleanest samples of the cuprates. Some impurity effects can help reveal whether the pseudogap has a superconducting origin and thus test various theories. Here we extend the pairing fluctuation theory for a clean system [Phys. Rev. Lett. 81, 4708 (1998)] to the case with nonmagnetic impurities. Both the pairing and the impurity $T$ matrices are included and treated self-consistently. We obtain a set of three equations for the chemical potential $μ$, $T_c$, the excitation gap $Δ(T_c)$ at $T_c$, or $μ$, the order parameter $Δ_{sc}$, and the pseudogap $Δ_{pg}$ at temperature $T<T_c$, and study the effects of impurity scattering on the density of states, $T_c$ and the order parameter, and the pseudogap. Both $T_c$ and the order parameter as well as the total excitation gap are suppressed, whereas the pseudogap is not for given $T\le T_c$. Born scatterers are about twice as effective as unitary scatterers in suppressing $T_c$ and the gap. In the strong pseudogap regime, pair excitations contribute a new $T^{3/2}$ term to the low $T$ superfluid density. The initial rapid drop of the zero $T$ superfluid density in the unitary limit as a function of impurity concentration $n_i$ also agrees with experiment.

cond-mat.supr-con↗
arXiv2000

Clapping modes in unconventional superconductors

We consider a superconducting state with a mixed symmetry order parameter components, e.g. $d+is$ or $d+id'$ with $d'= d_{xy}$. We argue for the existence of the new orbital magnetization mode which corresponds to the oscillations of relative phase $ϕ$ between two components around an equilibrium value of $ϕ= \fracπ{2}$. It is similar to the so called ``clapping'' mode in superfluid $^3He-A$. We estimate the frequency of this mode $ω_0(B,T)$ depending on the field and temperature for the specific case of magnetic field induced $d'$ state. We find that this mode is {\em tunable} with an applied magnetic field with $ω_0(B,T) \propto B \0$, where $\0$ is the magnitude of the d-wave order parameter. We argue also that similar filed induced clapping mode should be present in an organic p-wave superconductors.

cond-mat.supr-con↗
arXiv2000

Superconducting $d_{x^2-y^2} \pm id_{xy}$ phase glass

We discuss the effects of magnetic impurities on d-wave superconductors. We calculate the electron mediated RKKY interaction between the impurity spins in a d-wave superconductor and find that it decays as $r^{-3}$ at large distances. We argue that this interaction leads to the formation of a spin glass at low temperature $T \ll T_c$. It was previously shown that a local complex $Δ^1 \sim d_{xy}$ order parameter is induced around each impurity spin. We consider the pair tunneling resulting in the Josephson interaction between different patches of induced $d_{xy}$ order parameter. Due to the local coupling between impurity spins and the superconducting order parameter the Josephson coupling favors a ferromagnetic phase at low temperatures. The competition between the Josephson coupling and the RKKY interaction gives rise to an interesting phase diagram. At low impurity concentrations we find an unusual supercondcting phase glass, where the impurity spins $S^z$ and $d_{xy}$ component are disordered and yet the product of these two develops a true long range order $ $. This phase has no analog in purely magnetic spin glasses and arises as a result of the direct coupling of the impurity spin to the phase of $Δ^1$. At high impurity concentrations it is possible that a ferromagnetic phase will form.

cond-mat.supr-con↗
arXiv2000

Collective Mode in a d_{x^2-y^2} + id_xy Superconductor

We consider a superconducting state with a mixed symmetry order parameter components, e.g. d+is or d+id' with d'= d_{xy}. We argue for the existence of the new orbital magnetization mode which corresponds to the oscillations of relative phase ϕbetween two components around an equilibrium value of ϕ= π/2. It is similar to the so called ``clapping'' mode in superfluid 3He-A. We estimate the frequency of this mode ω_0(B,T) depending on the field and temperature for the specific case of magnetic field induced d'= d_{xy} state. We find that this mode is tunable with an applied magnetic field with ω_0(B,T) \propto B D_0, where D_0 is the magnitude of the d-wave order parameter. We also estimate the velocity s(B,T) of this mode.

cond-mat.supr-con↗
arXiv1998

New Mechanism of Quantum Oscillations in the Suprcpnducing Mixed State (Hc1<<B<<Hc2)

We argue that inhomogeneity inherent to the presence of periodic supercurrents in the vortex lattice sorts excitations by energies into the ones that are spatially localized and those that perform motion along large Larmour orbits. This energy threshold results in a new mechanism for the de Haas-van Alphen oscillations which enhances oscillations at B<<Hc2, even for an isotropic superconductor with a constant gap. We suggest that the mechanism is of a general character and can cause the slow decay of the de Haas-van Alphen effect when the field, B, decreases below Hc2.

cond-mat.str-el↗
arXiv1998

Topological Excitations of One-Dimensional Correlated Electron Systems

Properties of low-energy excitations in one-dimensional superconductors and density-wave systems are examined by the bosonization technique. In addition to the usual spin and charge quantum numbers, a new, independently measurable attribute is introduced to describe elementary, low-energy excitations. It can be defined as a number w which determines, in multiple of $π$, how many times the phase of the order parameter winds as an excitation is transposed from far left to far right. The winding number is zero for electrons and holes with conventional quantum numbers, but it acquires a nontrivial value w=1 for neutral spin-1/2 excitations and for spinless excitations with a unit electron charge. It may even be irrational, if the charge is irrational. Thus, these excitations are topological, and they can be viewed as composite particles made of spin or charge degrees of freedom and dressed by kinks in the order parameter.

cond-mat.supr-con↗
arXiv1998

Inhomogeneous States of Nonequilibrium Superconductors: Quasiparticle Bags and Antiphase Domain Walls

Nonequilibrium properties of short-coherence-length s-wave superconductors are analyzed in the presence of extrinsic and intrinsic inhomogeneities. In general, the lowest-energy configurations of quasiparticle excitations are topological textures where quasiparticles segregate into antiphase domain walls between superconducting regions whose order-parameter phases differ by $π$. Antiphase domain walls can be probed by various experimental techniques, for example, by optical absorption and NMR. At zero temperature, quasiparticles seldom appear as self-trapped bag states. However, for low concentrations of quasiparticles, they may be stabilized in superconductors by extrinsic defects.

cond-mat.supr-con↗
arXiv1997

Collective Excitations in High-Temperature Superconductors

Collective, low-energy excitations in quasi-two-dimensional d-wave superconductors are analyzed. While the long-range Coulomb interaction shifts the charge-density-wave and phase modes up to the plasma energy, the spin-density-wave excitation that arises due to a strong local electron-electron repulsion can propagate as a damped collective mode within the superconducting energy gap. It is suggested that these excitations are relevant to high-Tc superconductors, close to the antiferromagnetic phase boundary, and may explain some of the exotic features of the experimentally observed spectral-density and neutron-scattering data.

cond-mat.supr-con↗
arXiv1997

Systematic numerical study of spin-charge separation in one dimension

The problem of spin-charge separation is analyzed numerically in the metallic phase of the one-band Hubbard model in one dimension by studying the behavior of the single-particle Green's function and of the spin and charge susceptibilities. We first analyze the Quantum-Monte Carlo data for the imaginary-time Green's function within the Maximum Entropy method in order to obtain the spectral function at real frequencies. For some values of the momentum sufficiently away from the Fermi surface two separate peaks are found, which can be identified as charge and spin excitations. In order to improve our accuracy and to be able to extend our study to a larger portion of the Brillouin zone, we also fit our data with the imaginary-time Green's function obtained from the Luttinger-model solution with two different velocities as fitting parameters. The excitation energies associated with these velocities turn out to agree, in a broad range of momenta, with the ones calculated from the charge and spin susceptibilities. This allows us to identify these single-particle excitations as due to a separation of spin and charge. Remarkably, the range of momenta where spin-charge separation is seen extends well beyond the region of linear dispersion about the Fermi surface. We finally discuss a possible extension of our method to detect spin-charge separation numerically in two dimensions.

cond-mat.str-el↗
arXiv1997

Spectral Properties of Quasiparticle Excitations Induced by Magnetic Moments in Superconductors

The consequences of localized, classical magnetic moments in superconductors are explored and their effect on the spectral properties of the intragap bound states is studied. Above a critical moment, a localized quasiparticle excitation in an s-wave superconductor is spontaneously created near a magnetic impurity, inducing a zero-temperature quantum transition. In this transition, the spin quantum number of the ground state changes from zero to 1/2, while the total charge remains the same. In contrast, the spin-unpolarized ground state of a d-wave superconductor is found to be stable for any value of the magnetic moment when the normal-state energy spectrum possesses particle-hole symmetry. The effect of impurity scattering on the quasiparticle states is interpreted in the spirit of relevant symmetries of the clean superconductor. The results obtained by the non-self-consistent (T matrix) and the self-consistent mean-field approximations are compared and qualitative agreement between the two schemes is found in the regime where the coherence length is longer than the Fermi length.

cond-mat.supr-con↗
arXiv1995

Interactions for odd-omega gap singlet superconductors

A class of singlet superconductors with a gap function $Δ({\bf k}, ω_n)$ which is {\it odd} in both momentum and Matsubara frequency was proposed recently \cite{ba}. To show an instability in the {\it odd} gap channel, a model phonon propagator was used with the $p$-wave interaction strength larger than the $s$-wave. We argue that the positive scattering matrix element entering the Eliashberg equations leads to a constraint on the relative strength of $p$- and $s$-wave interactions which inhibits odd pairing. However, a general spin dependent electron-electron interaction can satisfy all constraints and produce the odd singlet gap. A possibility which may lead to an odd gap is a strongly antiferromagnetically correlated system, such as a high-$T_c$ material.

cond-mat↗
arXiv1995

Properties of Odd Gap Superconductors

We discuss the class of superconductors which have pairing correlations which are odd in frequency, as introduced originally by Berezinskii and more recently by Balatsky and Abrahams. As follows from the equations of motion, a natural definition of the thermodynamic order parameter of the odd-pairing state is the expectation value of a composite operator which couples a Cooper pair to a spin or charge fluctuation. We use a model pairing hamiltonian to describe properties of the odd-pairing composite-operator condensate. We show that the superfluid stiffness is positive, we discuss superconductive tunneling with an ordinary superconductor and we derive other thermodynamic and transport properties.

cond-mat↗
arXiv1993

Properties of Odd Gap Superconductors

A new class of superconductors with the gap function {\it odd} under time reversal is considered. Some of the physical properties of these superconductors such as the Meissner effect, composite condensate, gapless spectrum and transition from the {\it odd} gap superconductor to the BCS state at lower temperatures are discussed.

cond-mat↗